Interesting. You got me looking. Are you thinking about this? [www.smud.org]

Joining the Energy Imbalance Market is not quite the same as being subject to ISO control. The ISO does operate the market. But they still can't tell SMUD to shut off power in rolling outages like they can (and do) PG&E; that decision is still up to SMUD. It does give SMUD more incentive to conserve energy and sell surplus into the ISO's market, which seems to have grown so big that there's really no other market available in the West. And as I read that press release (and stuff at the EIM web site) SMUD didn't join by itself; it joined as part of a Balancing Authority of Northern California that it was already a member of - it was the BANC that joined the ISO's market.

As for who's part of the EIM, but I did finally find this PR piece: [www.youtube.com]
and this web page which is more informative: [www.westerneim.com]